Hi,

for those interested or for future reference, I traced my Fender Twin
resonant 1 KHz "ping" and other feedback strangeness to an O/C resistor to
ground on the HT circuit. HT was too high.

Of course in hindsight that should have been the first thing I checked
..........



Gareth.

"Gareth Magennis"

for those interested or for future reference, I traced my Fender Twin
resonant 1 KHz "ping" and other feedback strangeness to an O/C resistor to
ground on the HT circuit. HT was too high.



** That would be R129, a 30 kohms, 10 watt, cement wire wound resistor.

Its failure is quite common and causes a 10% rise in the low current DC
supplies - particularly to the the phase splitter.

The only reason it is there is to save the 450 volt filter electros from
overvoltage at first switch on.


Sorry - I do not buy your story.


BTW

Stop calling your amp a "Fender Twin" aka the famous "Twin Reverb" .

Its name is "The Twin" - aka the "Evil Twin" .





.......... Phil

"Phil Allison" wrote in message
...

"Gareth Magennis"

for those interested or for future reference, I traced my Fender Twin
resonant 1 KHz "ping" and other feedback strangeness to an O/C resistor
to ground on the HT circuit. HT was too high.



** That would be R129, a 30 kohms, 10 watt, cement wire wound resistor.

Its failure is quite common and causes a 10% rise in the low current DC
supplies - particularly to the the phase splitter.

The only reason it is there is to save the 450 volt filter electros from
overvoltage at first switch on.


Sorry - I do not buy your story.


Correct on the component ID. I've replaced this pair (2K7 as well) before
too, as they seem to fall apart. I must admit I did not go back and check
whether the resistor was actually causing the ringing. I found it open
circuit and replaced it, now the ringing and everything else has stopped,
and the amp sounds good. In fact I couldn't stop playing guitar through it.
I'm tempted to go back and disconnect it now though .....




Gareth.

Actually Phil, I should add that Fender do not have thes values in stock,
nor could I get them from my usual suppliers, so the 30K resistor is now 33K
and the 2K7 is now 3K3. I tried parallelling a 15K with the 3K3 to bring
it down to 2K7, but the HT got too high. With these values now, it sits at
393v instead of 397v going to 308v instead of 310v at the far end. Hence
my assumtion that the open circuit 30K would cause a rise in HT as a
potential divider circuit.


Gareth.

"Gareth Magennis"

Actually Phil, I should add that Fender do not have thes values in stock,
nor could I get them from my usual suppliers, so the 30K resistor is now
33K and the 2K7 is now 3K3.


** I have used 25 or 50 watt rated " metal clad " resistors to replace
that darn 30 kohm cement POS.

Bolted to the chassis and wired to the PCB - will outlast the amp.



I tried parallelling a 15K with the 3K3 to bring it down to 2K7, but the
HT got too high. With these values now, it sits at 393v instead of 397v
going to 308v instead of 310v at the far end.


** You are much too fussy about a few volts here or there.




........... Phil